The present invention relates to an information transmission system and, more particularly, to a communication network in which information processors are coupled to a common communication medium through MAUs (medium attachment units).
Recently, a fully distributed peer protocol transmission system, namely, CSMA/CD (carrier sense multiple access with collision detection) system has been developed as an advanced bus-shaped network. This system performs baseband transmission of information using a coaxial cable. On one hand, broadband networks are getting most of the attention for efficiently utilizig the coaxial cable.
In a broadband network using an FSK modulation system, it is effective to detect the level of a beat signal which is generated due to a collision for a collision detection system to realize the CSMA/CD system. To effectively detect the collision, it is necessary to adjust the output level of a transmission signal of each MAU so that the level of the transmission signal from each MAU becomes equal at one point of the communication medium, for instance, at a headend at which a transmission path of the communication medium is coupled to a reception path, in other words, so that each MAU can receive signals from all MAUs at an equal level. This level adjustment is accomplished by adjusting the gain of a transmission amplifier of each MAU.
A gain adjustment system of the transmission amplifier of each MAU, which allows an effective detection of a signal collision on a broadband network, is disclosed in a copending U.S. patent application Ser. No. 563,241, entitled "INFORMATION TRANSMISSION SYSTEM WITH MODEMS COUPLED TO A COMMUNICATION MEDIUM" filed Dec. 12, 1983 and assigned to the same assignee as this application. According to one system disclosed in this application, a reference level pilot signal is sent to the headend of the communication medium and the reception level of the pilot signal is detected in each MAU and then the gain of a reception amplifier is adjusted in accordance with this reception level so that the reception level suitable for the receiving process is obtained. Thereafter, each MAU transmits a test signal at each predetermined timing and adjusts the gain of the transmission amplifier such that the reception level of this test signal becomes equal to the reception level of the pilot signal, in other words, such that the level of the test signal becomes, at the headend, equal to the level of the pilot signal.
In the case of such a gain adjustment system, it is required that the test signal transmission timing of the MAUs be different from one another. To this end, each of the MAUs is provided with dip switches to set the transmission timing of the test signal. However, to prevent simultaneous transmission of the test signals from MAUs, it is necessary to control the test signal transmission timings of MAUs over the network. In the case where the number of MAUs on the network increases, the test signal transmission timing of each MAU must be readjusted.